The invention relates to a process for treating tantalum and/or niobium containing raw materials such as wastes, slags, concentrates, and ores. The invention also relates to regeneration of fluoride ions and ammonium and to recycling them into the process of the invention. At the same time particular account is taking of environmental protection by eliminating harmful waste materials.
Decomposition methods of tantalum and niobium containing raw material in form of, for example, ores, slags and concentrates received from them, or wastes of any origin by chemical and/or metallurgical processes are known.
According to the known chemical processes, tantalum and niobium containing materials are dissolved in mineral acids by adding hydrofluoric acid and by simultaneous heating. Solutions containing niobium and tantalum, in form of their complex fluoro acids and loaded with raw material still in firm condition after the decomposition process and other impurities are received.
The selective separation of fluoride components of tantalum and niobium takes place according to known methods by multistage liquid extraction, during which simultaneously, during re-extraction from organic phase, separation of fluoro acids of tantalum and niobium is achieved by choice of a suitable concentration of hydrofluoric acid in aqueous phase. Such processes are described, for example, in the U.S. Pat. No. 3,117,833, U.S. Pat. No. 2,953,453 and U.S. Pat. No. 2950966. According to a known process described in the patent DE 4207145 tantalum is extracted as its fluoro acid from an aqueous solution of niobium fluoro acid.
Proceeding from the pure aqueous solutions of the complex tantalum and niobium fluoro acids in the known methods, tantalum and niobium are obtained as oxides in pure form by precipitating them from aqueous solutions as fluoride containing oxide hydrates by adding ammonia and transformation of the received oxide hydrates into pure oxides by calcining at higher temperatures.
Naturally, large amounts of waste waters containing ammonium fluoride accumulate in this known process. This either leads to environmental pollution or requires special steps in order to recover ammonia by reaction with calcium lime, and to deposit the fluoride in a harmless form as a deposable fluorite slurry.
Besides, the known process requires for its realization constant use of fresh hydrofluoric acid in a volume necessary for dissolution of tantalum and niobium containing raw materials and also for realizing separation extraction. The obtained solution is exposed to multistage liquid extraction, which provides complete extraction of tantalum and niobium from the raffinate solution. However, it does not guarantee the required quality of the tantalum and niobium compounds.
In order to get the required quality of the tantalum and niobium compounds their aqueous solutions of complex fluoro acids undergo additional purification by repeated multistage liquid extraction, and it, in turn, results in an increase of costs for expensive chemicals and an increase of costs for neutralization of liquid wastes.
According to the known process, precipitation of oxide hydrates of tantalum and niobium is carried out well enough while the obtained solutions of ammonium fluoride are wastes and no extraction from them of tantalum and niobium is made. The given precipitation is made in conditions, when all mixture elements are co-precipitated together with deposits of oxide hydrates of tantalum and niobium.
In the present invention the usual dissolution of tantalum and/or niobium containing materials in mineral acids is avoided together with all its defects. At the same time fluoride ions and ammonia are recovered and recycled into the process. The extraction raffinates are repeatedly used as employable liquid for separation of tantalum and niobium by multistage liquid extraction.
The purpose of the present invention is to eliminate or reduce the above drawbacks. A further purpose is to achieve a less expensive process. Additional purposes and improvements will be appreciated by a person skilled in the art when reading the detailed description below.
In the description all stated percentages are weight-%, if not explicitly stated otherwise.
The tantalum and niobium containing raw material is processed by use of a solution of ammonium fluoride recovered from the process at a temperature up to the boiling point (239xc2x0 C.) for not more than 10 hours (examples 1-4).
It is not expedient to increase the temperature and to increase the time of decomposition. If said temperature is increased and the time of treatment is more than 10 hours it leads to intensive extraction of ammonium fluoride out from the reaction zone and into a gas phase. This will firstly complicate the process of gas cleaning and, secondly, it will require increased amounts of ammonium fluoride for decomposition of the raw materials.
The necessary quantity of ammonium fluoride for decomposition of tantalum and niobium raw materials is calculated based on formation of ammonium fluorite complex compounds of all elements which are included in the structure of the given raw materials. In order to achieve full decomposition of tantalum and niobium raw materials it is expedient to use a surplus of ammonium fluoride. Thus in examples 1-4 ammonium fluoride is used in a surplus of 1.5-2.5 times. However, this surplus is not compulsory and is only given as an example.
At the stage of decomposition ammonia and ammonium fluoride are transformed into gas phase.
The ammonia is formed while the elements of oxides that the raw materials contain undergo fluoridation, and also in the process of transition of ammonium fluoride to ammonium hydrofluoride when the solution of ammonium fluoride is evaporated during the process of decomposition.
The ammonium fluoride is transformed into gas phase mainly owing to thermal decomposition of ammonium fluoride complex compounds of mixture elements formed during the treatment of the raw materials.
From gas phase ammonia is recovered as 1.0-28% aqueous solution by the method of condensation (solutions (1.4), (1.6), (2.4) and (2.6) in examples 1 and 2) and it is used repeatedly during ammoniac precipitaion (for example 20-28% aqueous solution) and during scrubbing (for example 1.5% aqueous solution) of tantalum or niobium containing precipitates (examples 7 and 8), and also during scrubbing of the sediments of the mixture elements (examples 3 and 4).
A 1.0-1.5% aqueous solution of ammonia (for example solution 2.4 from example 2) as well as circulating water can be used during the operation of leaching (example 4).
The ammonium fluoride is recovered from gas phase by the method of condensation and absorption of an aqueous solution containing ammonium fluoride (solutions 1.5 and 2.5 in examples 1 and 2) in an amount of about 200 g/l.
At the stage of decomposition of tantalum and niobium containing raw materials, the mixture elements, contained in the raw materials, are extracted like almost insoluble oxides, oxide fluorides or fluorides (precipitates 1.3; 2.3; 3.3 and 4.3 in examples 1-4), which after calcining are deposable waste products (example 1).
At the stage of decomposition and leaching aqueous solutions are obtained containing fluoro tantalum and fluoro niobium complexes of ammonium (solutions 1.1; 1.2; 2.1; 2.2; 3.1; 3.2; 4.1 and 4.2 in examples 1-4).
The scrub solutions containing considerable quantities of mixture elements (for example solutions 1.2 and 2.2 in examples 1 and 2) are expedient to use at the stage of decomposition of tantalum and niobium containing raw materials as a solution of ammonium fluoride (example 4).
From pure tantalum and niobium containing aqueous solutions (for example 1.1; 2.1; 3.1 and 4.1 in examples 1-4) the mixture of fluorides and oxide fluorides of tantalum and niobium is consistently extracted into solid condition:
by ammoniac precipitation of ammonium fluoride complex salts of tantalum and niobium with the help of circulating 20-28% ammonia solution;
by scrubbing the allocated salts.with water or 1.0-1.5% ammonia solution (it is expedient to use water for lowering the hydrolysis of fluoride salts); and
by drying/calcination of ammonium fluoride complex salts of tantalum and niobium with the purpose of decomposition of ammonium complexes up to full removal of ammonia from the solid phase (examples 5 and 6).
The precipitation of tantalum and niobium like their fluoride-containing salts from aqueous solution can be carried out in conditions of complete precipitation (example 5), and in conditions of incomplete precipitation (example 6). It is more preferable to use the conditions of incomplete precipitation because then the amount of ammonia used for precipitation is lower and the obtained precipitates contain more fluoride ions relatively to the sum of tantalum and niobium.
Drying/calcination is carried out at a high temperature not higher than 450xc2x0 C. during not more than 2 hours.
It is not expedient to increase the temperature or the time, because if the temperature is higher than 450xc2x0 C. and the time of treatment is longer than 2 hours sublimation of fluorite compounds of tantalum and niobium into gas phase is noticeable, and this lowers the direct extraction of tantalum and niobium and thus leads to a rise in the costs of the process as a whole.
The thus obtained mixture of fluorides and oxide fluorides of tantalum and niobium (sediments 5.2 and 6.2 in examples 5 and 6) is treated by reused extraction raffinate, that does not contain considerable quantity of mixture elements, in order to get the solution deposable for selective separation of fluoro compounds of tantalum and niobium (examples 7 and 8).
Accumulation of mixture elements at the stage of extraction separation of tantalum and niobium is excluded by extracting approximately 1/20-1/25 parts of extracted raffinate from the extraction system, and at the same time the part extracted from the system raffinate is used in a technological process for clearing ammoniac gas emissions (examples 7 and 8).
The stage of extraction separation (examples 7 and 8) is carried out according to known methods (for example patents DE 402107, U.S. Pat. No. 5,209,910 and DE 4207145 without use of a sulphuric acid) and pure aqueous solutions containing fluoroacid complexes of niobium (for example solution 7.1) and fluoro ammonium complexes of tantalum (for example solution 7.2) are obtained.
Oxides of tantalum (7.2.1) and niobium (7.1.1) are released in pure form from the obtained solutions by precipitation as fluoride containing oxide hydrates by adding; ammonia and transformation the thus obtained oxide hydrates into pure oxides by calcining at high temperature (example 7).
In the process the solutions of ammonium fluoride are formed:
by condensation of a gas phase at the stage of decomposition of tantalum and niobium ore (solutions (1.5) and (2.5) in examples 1 and 2);
in a filtrate after ammoniac precipitating of fluoride containing tantalum and niobium sediment (5.2) and (6.2) from aqueous solutions (1.1), (2.1), (3.1), (4.1), containing fluoro tantalum and fluoro niobium complexes of ammonium, (examples 5 and 6);
in a filtrate after oxide hydrates of tantalum and niobium are precipitated by solutions of ammonia from tantalum and niobium re-extracts (solutions (7.1.2), and (7.2.2) in example 7); and
a clearing of the gas phase from ammonia compounds by using approximately 1/20-1/25 parts of a fully extracted raffinate solution (for example solution (7.3) in example 7) for the operations:
when tantalum and niobium containing ores are decomposed;
when fluoride containing sediment of the mixture elements are being dried and calcinated;
when fluoride containing tantalum and niobium sediment are being dried and calcinated; and
when oxides of tantalum and niobium are being dried and calcinated.
All selected solutions of ammonium fluoride are reused directly at the stage of decomposition of the tantalum and niobium containing raw material, or at the stage of scrubbing the sediment of mixture elements.
In the process of the invention there is a loss of fluoride ions with the sediment of mixture elements, the quantity of which is not more than 25% of the whole quantity of fluoride ions necessary for decomposition of the raw materials. The quantity of the losses of fluoride ions depends on the element structure of the impurities in the initial tantalum and/or niobium containing raw materials. The losses of fluoride ions in the process is compensated by adding fresh 40-70% hydrofluoric acid at the stage when the solutions for extraction separation of tantalum and niobium is being prepared, or at the stage of extraction when the scrubbing solutions are being prepared (examples 7 and 8) or also by adding fresh ammonium fluoride salts in solid form or dissoluted at the stage of decomposition of the tantalum and/or niobium containing raw materials (examples 1-4).